Two stroke spark ignition engines, although they are being constantly developed are recognised as suffering from the certain disadvantages, such as:
Excessive oil consumption. This is because it is necessary to mix oil with the petrol prior to carburation or to inject the lubricating oil directly into the induction port to provide adequate lubrication to the moving parts of the engine. Because only a small proportion of the oil within the petrol/oil mixture actually reaches the areas of the engine that require lubrication, more oil than would otherwise be necessary to ensure adequate lubrication must be mixed with the petrol. Consequently two stroke engines are prone to excessive exhaust pollution through smoke.
A further disadvantage results from the usual construction whereby the intake and exhaust of gases into and out of the cylinder is arranged through ports in the cylinder wall, with the ports being successively covered and uncovered during the reciprocating movement of the piston. To obtain adequate gas flow, the ports are necessarily comparatively large in area and this presents problems in excessive wear in both the piston rings and in the skirt of the piston below the piston rings.
A yet further disadvantage with the known porting arrangements is that the gas path through the cylinder area is difficult to optimise to obtain optimum combustion.
A still further disadvantage is that to obtain satisfactory scavenging of the combustion gases, the positioning of the transfer and exhaust ports has to be arranged so that a significant portion of the incoming charge is mixed with the outgoing combusted gases and this leads to inefficiencies.